The Trimap is not the result of computation, it is something hand-painted by me, using three colours. The mask/selection is generated by the program, but not the Trimap.
In ReMask, the Trimap could be saved before computing the mask. Something which I did as a routine part of my workflow.
More to the point, the dialog finishes with a confirmation that the file has been saved, despite the fact that nothing happens. Try it!
I’m sure the programmers would like to deal with this.